Pulmonary surfactant protein is an apo-protein specific to pulmonary surfactant (SP). Hitherto, there have been reported four kinds of pulmonary surfactant protein; i.e., hydrophilic SP-A and SP-D and hydrophobic SP-B and SP-C. Among four kinds of protein, SP-D is considered to play an important role in the defense mechanism in the respiratory tract-alveolus system. It has been reported that pulmonary diseases such as idiopathic interstitial pneumonia can be diagnosed by assaying SP-D of a sample (e.g., serum) through an immunological technique employing antigen-antibody reaction (Non-Patent Document 1).
The above-reported method employs recombinant SP-D (rSP-D) produced through recombinant DNA technology as a standard substance. Although the rSP-D has poor stability, it has also been reported that the stability of pulmonary surfactant proteins including rSP-D can be enhanced through causing an ion of a metal belonging to the sub-group 2-a of a periodic table such as calcium, barium, or magnesium to coexist. (Patent Document 1).
Non-Patent Document 1: Igaku & Yakugaku, 36(4), 803-808 (1996)
Patent Document 1: Japanese Patent No. 3573330